Structural-Phase State and Magnetotransport Properties of
Transcript Of Structural-Phase State and Magnetotransport Properties of
JOURNAL OF NANO- AND ELECTRONIC PHYSICS Vol. 13 No 1, 01020(6pp) (2021)
ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Том 13 № 1, 01020(6cc) (2021)
Structural-Phase State and Magnetotransport Properties of Thin Film Alloys Based on Permalloy and Copper
I.O. Shpetnyi1,*, K.V. Tyschenko1, V.Ya. Pak1, V.I. Duzhyi2, Yu.O. Shkurdoda1, I.Yu. Protsenko1
1 Sumy State University, 2, Rimsky-Korsakov St, 40007 Sumy, Ukraine 2 National Aerospace University "Kharkiv Aviation Institute", 17, Chkalov St, 61070 Kharkiv, Ukraine
(Received 03 January 2021; revised manuscript received 15 February 2021; published online 25 February 2021)
The paper presents the results of research influence of composition on structural-phase state and magnetoresistive properties of as-deposited and heat-treated at temperatures Ta ≤ 900 K samples film alloys based on Py and Cu. Samples of thin film alloys with a thickness of d 25 nm in the range of 19 ≤ cCu ≤ 69 (where cCu is the concentration of Cu, at.%) were obtained by the method of co-evaporation in vacuum from two independent evaporators. The structural-phase state of the samples of film alloys at cCu 19 at.%, 34 at.% and 61 at.% was investigated by the method of transmission electron microscopy. The structure of thin films in both as-deposited and annealed at Ta ≤ 900 K state consists of quasi granules with permalloy embedded in a nonmagnetic Cu matrix. The phase state of the samples at cCu 19 at.% and cCu 34 at.% in the as-deposited state and after heat treatment at Ta 600 K corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu. After heat treatment at temperatures of 700 Ta 900 K, the phase state of the samples at cCu 19 at.% and cCu 34 at.% corresponded to Ni3Fe + fcc-Cu. For the film alloy sample at cCu 61 at.% in the as-deposited state and after heat treatment at temperatures of 600 Ta 900 K, the phase state corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu. Studies of the magnetoresistive properties of film samples showed that the film samples in the entire range of compositions of 19 ≤ cCu ≤ 69 at.% were characterized by isotropic magnetoresistance. The maximum value of the giant magnetoresistance was observed for the sample with cCu 34 at.% Both in the as-deposited state and after heat treatment at temperatures of 600 Ta 900 K. Heat treatment of samples in the temperature range of 600 Ta 900 K had almost no effect on the value of GMR films at 19 ≤ cCu ≤ 51 at.%.
Keywords: GMR effect, Granular state, Spin-dependent scattering, Permalloy, Superparamagnetism.
DOI: 10.21272/jnep.13(1).01020
PACS numbers: 61.05.J –, 61.72. – y, 75.47.De, 81.40.Gh
1. INTRODUCTION
Thin metal film materials are characterized by a number of phenomena, the study of which remains relevant today [1-6]. One such effect is the phenomenon of giant magnetoresistance (GMR) in multilayers and granular film alloys [7-10]. The discovery of the GMR phenomenon has aroused widespread interest in these nanostructures due to the possibility of their application in spintronics, sensor electronics, biotechnology, medicine, instrumentation and other fields [1, 2, 11-15]. The effect of giant magnetoresistance is observed in granular magnetic films based on alloys of Co-Ag, Fe-Ag, Co-Cu, Permalloy-Ag, Permalloy-Cu, where magnetic granules with a size of units up to 100 nm are randomly placed in the volume of the nonmagnetic matrix [13, 14, 16-18]. The GMR effect is the result of spin-dependent scattering of conduction electrons at the interface between the nonmagnetic matrix and the magnetic granule or in the volume of the magnetic granule [6, 17]. When making such samples in a magnetic field, their resistivity shows large changes. It is established that the physical properties of granular magnetic film materials are determined by their composition and, accordingly, the structural-phase state. The magnetic, magnetoresistive, magneto-optical and electrical properties can be controlled by the change of the film alloy composition, and accordingly affecting the size and concentration of magnetic granules in the nonmagnetic matrix [16, 19]. Nowadays, the efforts of researchers are aimed at developing new film systems
with specified physical properties and ensuring the stability of these properties under the action of various factors, including heat treatment.
To date, a large number of experimental results of the study of magnetic and magnetoresistive properties of multilayers and multilayer film systems based on permalloy (Py) and Cu [17, 20-23]. Py and Cu-based film systems are widely used due to certain unique features low values of coercivity and saturation magnetization [1], thermal stability [21, 22], high sensitivity to the magnetic field [20]. Due to these properties, magnetic film systems have found practical application in biomedical technologies in the manufacture of biosensors [1], in automobile electronics [1], in spintronics in the manufacture of sensitive elements of magnetic field sensors [2], magnetic transducers, magnetoresistive random access memory [24]. In the works [21, 22] the authors studied the effect of annealing of multilayers based on permalloy and copper on their magnetic and magnetotransport properties, analyzed the mechanism of degradation of samples. However, studies of these effects for thin-film alloys in a wide range of compositions based on Py and Cu, obtained by the method of simultaneous condensation of two independent evaporators in the literature are almost absent. The aim of this work was to study the influence of the composition of film samples based on Py and Cu with a thickness d 25 nm on their structural-phase state and magnetoresistive properties under heat treatment.
* [email protected] 2077-6772/2021/13(1)01020(6)
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2021 Sumy State University
I.O. SHPETNYI, K.V. TYSCHENKO, V.YA. PAK ET AL.
2. EXPERIMENTAL DETAILS
Samples of thin-film alloys based on permalloy and
copper in a wide range of 19 ≤ сCu ≤ 69 (where сCu is the
total copper concentration, at.%) were obtained in a
vacuum of P ≈ 10 – 4 Pa at room temperature. To obtain
these film samples used the method of simultaneous
evaporation of metals from two independent electron
beam evaporators. The starting material for spraying
Py was a massive portion of Ni80Fe20 permalloy. The
deposition rate was 0.2 nm/s for Py and 0.3 nm/s for
Cu. The thickness of the film samples during the depo-
sition process was controlled by the quartz resonator
method [24]. Polished sieve substrates were chosen for
deposition. All the obtained samples were obtained in
one deposition cycle, therefore, they had the same
thickness d 25 nm, but differed in the ratio of the
concentration of the components. Annealing of samples
at temperatures Ta 600, 700 and 900 K was carried
out in a vacuum chamber at P 10 – 4 Pa for a time
t 30 min. The elemental composition of the obtained
thin film samples was examined using scanning elec-
tron microscopy (SEM Jeol 7001TTLS) and energy-
dispersion X-ray (EDX) spectroscopy (Oxford Instru-
ments analyzer, XMax detector). An accelerating volt-
age of 10 kV and an operating distance of 10 mm were
used to obtain EDX spectra.
The structural-phase state of the samples was in-
vestigated by transmission electron microscopy (TEM)
on the device TEM-125K (accelerating voltage 100 kV).
Preparation of samples for research by the TEM meth-
od was carried out by deposition of permalloy and cop-
per on a grid with a carbon film. Samples for research
by TEM were obtained simultaneously with the sam-
ples for the study of magnetic transport properties, ie
were identical.
The study of magnetoresistive properties was car-
ried out at room temperature on an automatic measur-
ing system in two geometric dimensions – transverse
and longitudinal [26]. Measurements were performed
by applying a magnetic field with an amplitude in the
range Bmax ± 0.45 T using a 4-point measurement
scheme. In the transverse geometry of the measure-
ment, the applied magnetic field was directed in the
plane of the film, but perpendicular to the direction of
the current I. In the longitudinal geometry of the
measurement, the magnetic field was directed in the
plane of the film and parallel to the direction of the
current I [25]. GMR values were calculated by the ratio
GMR ∆R/R(Bmax) (R(B) – R(Bmax))/R(Bmax)
[20],
where R(B) and R(Bmax) are the resistance of the films
in an arbitrary magnetic field B and in the maximum
field Bmax, respectively.
3. RESULTS AN DISCUSSION
3.1 Structural State and Phase Composition
The results of studies of the structural-phase state of film samples are very important for the interpretation of their magnetoresistive properties. The structural-phase state of samples of film alloys with copper concentrations of 19 at.%, 34 at.% and 61 at.% was analyzed by TEM method in as-deposited state and after
J. NANO- ELECTRON. PHYS. 13, 01020 (2021)
heat treatment. It should be noted that according to [27], in the massive and film states in permalloy alloys, depending on the ratio between the concentrations of Ni and Fe, three crystalline phases can be formed. At cNi 6385 at.%, the phase composition of Py films corresponds to fcc-Ni3Fe (structural type Cu3Au) with the lattice parameter a 0.3540.359 nm. At concentrations of cNi 50 at.% in the films, the fcc-Ni-Fe phase (structural type CuAu) with the lattice parameter a 0.3590.361 nm is stabilized. At cNi 25, the bcc -Fe-Ni phase with the lattice parameter a 0.286 nm is stabilized.
In Figs. 1 and 2 microstructure images and diffraction spectra are presented for a film sample with a to-
tal copper concentration cCu 19 at.% in as-deposited
state and after heat treatment at 600 and 700 K. On the inserts Fig. 1a, b the corresponding diffraction patterns for film samples are given. The diffraction pattern was transformed into a spectrum, by software created in the programming language Lab View. On Fig. 2 diffraction spectra for the images of diffraction pattern presented on Fig. 1. Vertical lines on Fig. 2 indicate the tabular data of the diffraction lines for massive samples of fcc-Cu, fcc-Ni-Fe and fcc-Ni3Fe. The decoding of diffraction patterns was performed taking into account the positions of the peaks in the diffraction spectra for as-deposited and heat-treated samples.
Fig. 1 – Microstructure of thin-film alloy based Py and Cu with a concentration of copper cCu 19 at.% in as-deposited state (a) and after heat treatment at 700 K (b). The inserts show the corresponding diffraction patterns
Fig. 2 – Diffraction spectra from samples of alloy thin films based on Py and Cu with a total concentration of cCu 19 at.% in as-deposited state and after heat treatment at different temperatures Ta
Analysis of the research results showed that the phase state of this sample in the as-deposited state corresponds to fcc-NixFe (x ≈ 3) + fcc-Cu. The size of the quasi granules (G) permalloy was L 7÷18 nm (Fig. 1a). Lines of fcc-Cu and fcc-NixFe, cannot be separated on the diffraction pattern due to close interplanar distances. Deviations from the composition Ni3Fe to NixFe (x ≈ 3) may caused
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by the dissociation of Ni3Fe permalloy to Ni and Fe atoms during the process of electron beam evaporation. A phase can be formed from these atoms fcc-Ni-Fe. In addition, the diffusion of Ni and Fe atoms may result in the formation of dilute solid solutions fcc-Cu (Ni) and fcc-Cu (Fe). After heat treatment of the film sample with the total concentration of copper cCu 19 аt.% at Ta 700 K the phase state corresponded fcc-Cu + fcc-Ni3Fe. During heat treatment of samples due to thermal diffusion of Ni atoms and their attachment to the granules NixFe Ni3Fe stoichiometry is renewed as in the material of the original sample. The size of the quasi granules was L 7÷23 nm (Fig. 1b).
Structural-phase state of the film sample with the total concentration of copper cCu 34 аt.% in the asdeposited state and after heat treatment (Fig. 3 and 4) similar to the state of the sample at cCu 19 at.%. Asdeposited films were characterized by a state in which quasi granules fcc-(NixFe) (x ≈ 3) size L 7÷15 nm (Fig. 3a) were in a nonmagnetic matrix with fcc-Cu. The results of decoding diffraction patterns are presented in Table 1. Annealing at Ta 600 K did not cause changes in phase state (Fig. 3b). The size of the quasi granules was L 8÷16 nm. After heat treatment of the film samples at Ta ≥ 700 K, the phase state corresponded fcc-Ni3Fe + fcc-Cu. Annealing of the samples at Ta 700 K and Ta 900 K increased the size of the quasi granules with permalloy to L 8÷21 nm (Fig. 3c) and L 27 ÷ 43 nm, respectively.
magnetic matrix. The size of the granules in asdeposited and heat-treated at 600, 700 and 900 K states were L 6÷10 nm (Fig. 1a), L 8÷12 nm (Fig. 1b), L 8÷19 nm (Fig. 1c) and L 8÷23 nm (Fig. 1d), respectively.
Fig. 4 – Diffraction spectra from samples of alloy thin films based on Py and Cu with a total concentration of cCu 34 at.% in as-deposited state and after heat treatment at different temperatures Ta
Fig. 3 – Microstructure of thin-film alloy based Py and Cu with a concentration of copper cCu 34 at.% in as-deposited state (a) and after heat treatment at 600 (b), 700 (c) and 900 K (d). The inserts show the corresponding diffraction patterns
Analysis of the phase composition of the film sample at cCu 61 at.% in as-deposited state and after heat treatment at temperatures of 600 Ta 900 K corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu (Fig. 5). The results of decoding the diffraction spectra (Fig. 6) are given in Table 1. No renewal NixFe to the stoichiometry of Ni3Fe occurred during heat treatment of the samples caused by to the thermal diffusion of Ni atoms, possibly because of the low total concentration of the magnetic component and, accordingly, the Ni atoms in the sample.
The structures of thin films in both as-deposited and annealed at Ta 900 K states consist of permalloy fcc-NixFe quasi granules embedded in a Cu non-
Fig. 5 – Microstructure of thin-film alloy based Py and Cu with a concentration of copper cCu 61 at.% in as-deposited state (a) and after heat treatment at 600 (b), 700 (c) and 900 K (d). The inserts show the corresponding diffraction patterns
3.2 Magnetoresistive Properties
The results of the study on the magnetoresistive properties in as-deposited state and after heat treatment alloys thin films based on Py and Cu are shown below. On Fig. 7 the dependence of the GMR value on the total concentration of copper in the film alloy is given. Magnetoresistance studies were performed in the transverse (Fig. 7a) and longitudinal (Fig. 7b) measurement geometries at room temperature in an external magnetic field B 0.45 T. At low total concentrations of copper in the alloy (for example, when cCu 19 at.% and cCu 26 at.%) quasi granules with permalloy, can touch each other, form a normal ohmic conduction channel. Therefore, in this case, the spindependent scattering of conduction electrons is inefficient, and the amplitude of the GMR is correspondingly small (GMRmax ≈ 0.1 % in the as-deposited state for the sample with cCu 19 at.%).
The increasing total copper concentration to cCu 34 at.% rises the amplitude of GMRs up to 0.21 %,
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Fig. 6– Diffraction spectra from samples of alloy thin films based on Py and Cu with a total concentration of cCu 61 at.% in as-deposited state and after heat treatment at different temperatures Ta
the maximum value for as-deposited samples. In Fig. 8 field dependences for a sample of a film sample on the basis of permalloy and copper at the general concentration of copper are resulted cCu 34 at.%. The lack of saturation on the field dependences of the magnetoresistance (Fig. 8) indicates that the structural state of this sample corresponds to the superparamagnetic granules placed in a nonmagnetic matrix. Heat treatment of a sample with a total concentration of copper
cCu 34 at.% at temperature Ta 600 K led to an increase in the value of GMRs by 38 % and 14 % of the value obtained for as-deposited samples in the transverse and longitudinal geometry of the measurement, respectively. However, further heat treatment of the sample in the temperature range of 600 Ta 900 К almost did not change the value of its magnetoresistance (Figs. 7 and 8).
Further increase total copper concentration to 51 69 at.% caused reducing the amplitude of the magnetoresistance to 0.02-0.05 % in an external magnetic field of 0.45 T. The field dependences of the magnetoresistance of such samples are linear, do not show hysteresis and are not saturated in fields up to 0.45 T. This behavior of the magnetoresistance indicates that the structural state of these films corresponds to the ensemble of weakly interacting superparamagnetic granules. This is confirmed by the results of the study of the structural state, performed by the TEM method.
It should be noted that for film samples with a total concentration of copper 19 cCu 51 at.% the thermal stability of the magnetoresistance was observed. This feature of the magnetic transport properties of film systems based on Py and Cu can be used in the manufacture of instrument structures. Thermal stability of magnetoresistive properties was also observed by the authors in multilayers based on permalloy and copper [22].
Table 1 – Deciphering the diffraction patterns from Py and Cu alloy films with concentration of Cu 34 at.% and 61 at.% in asdeposited state and after heat treatment
No I,
dhkl,
hkl Phase state No I,
dhkl,
hkl Phase state
%
nm
%
nm
cCu 34 at.%
As-deposited sample
After annealing at Ta 900 K
1 100 0.2066 111 fcc-Cu G. fcc-NixFe
1 100 0.2040 111 fcc-Cu G. fcc-Ni3Fe
2 40 0.1808 200 fcc-Cu
2 65 0.1774 200 fcc-Cu
G. fcc-NixFe
G. fcc-Ni3Fe
3 20 0.1268 220 fcc-Cu
3 40 0.1251 220 fcc-Cu
G. fcc-NixFe
G. fcc-Ni3Fe
4 20 0.1084 311 fcc-Cu
4 40 0.1070 311 fcc-Cu
G. fcc-NixFe
G. fcc-Ni3Fe
a 0.359 ± 0.001 nm
a 0.355 ± 0.001 nm
cCu 61 at.%
As-deposited sample
After annealing at Ta 700K
1 100 0.2076 111 fcc-Cu G. fcc-NixFe
2 30 0.1808 200 fcc-Cu G. fcc-NixFe
3 20 0.1269 220 fcc-Cu G. fcc-NixFe
4 20 0.1089 311 fcc-Cu G. fcc-NixFe
a 0.360 ± 0.001 nm
1 100 0.2072 111 fcc-Cu G. fcc-NixFe
2 45 0.1798 200 fcc-Cu G. fcc-NixFe
3 30 0.1269 220 fcc-Cu G. fcc-NixFe
4 35 0.1083 311 fcc-Cu G. fcc-NixFe
a 0.359 ± 0.001 nm
Table values dhkl, nm
fcc-Cu 0.2080
fccNi-Fe
0.2060
fccNi3Fe
0.2044
0.1798 0.1783 0.1772
0.1271 0.1259 0.1253
0.1083 0.1073 0.1069
fcc-Cu 0.2080 0.1798 0.1271 0.1083
fccNi-Fe 0.2060
0.1783
0.1259
0.1073
fccNi3Fe 0.2044
0.1772
0.1253
0.1069
It should be noted that studies of the magnetoresistive properties of the samples showed that film alloys based on Py and Cu in the entire studied range of compositions 19 ≤ сCu ≤ 69 at.% In the as-deposited state and after heat treatment to Ta 900 K were characterized by an isotropic magnetoresistance. It means, the behavior of the magnetoresistance curves obtained in the transverse and longitudinal geometries of the measurement had a similar character.
4. CONCLUSIONS
1. The correlation between the composition, structural-phase state and magnetoresistive properties of samples of film alloys based on Py and Cu with a thickness of d 25 nm in the as-deposited state and after heat treatment at Ta 900 K was experimentally confirmed.
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Fig. 7 – The magnetoresistance in transverse (a) and longitudinal (b) geometries of nanogranular alloy thin films based on Py and Cu as a function of Cu atomic concentration, in as-deposited state and after annealing at different temperatures Ta. The measurements were performed at room temperature in magnetic field Bmax 0.45 T
Fig. 8 – The magnetoresistive curves measured in transverse (a) and longitudinal (b) geometries at the room temperature for the as-deposited and annealed at different temperatures nanogranular alloy thin films based on Py and Cu with a total concentration of cCu 34 at.%
2. The structural-phase state of samples of film alloys at сCu 19 at.%, 34 at.% and 61 at.% was investigated by TEM method. The structure of film systems in the as-deposited state and after heat treatment at
T 900 K consists of quasi-permalloy granules embedded in a nonmagnetic Cu matrix. Phase state of samples at cCu 19 at.% and cCu 34 at.% in as-deposited state and after heat treatment at Ta 600 K corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu. After heat treatment at temperatures 700 Ta 900 K phase state of the samples with concentrations of cCu 19 at.% and cCu 34 at.% corresponded to Ni3Fe + fcc-Cu. For a sample of a film alloy at cCu 61 at.% In the asdeposited state and after heat treatment at temperatures of 600 Ta 900 K phase state corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu.
3. Studies of the magnetoresistive properties of the measured samples showed that film systems based on Py and Cu in the entire range of compositions 19 ≤ сCu ≤ 69 at.% are characterized by isotropic magnetoresistance. The maximum value of the giant magnetoresistance was observed for a sample with a total copper concentration сCu 34 at.% as in as-deposited condition and after heat treatment at temperatures 600 Ta 900 K.
4. Heat treatment of film samples with total concentration copper 19 ≤ сCu ≤ 51 at.% in the temperature range 600 Ta 900K had almost no effect on the value of GMRs. It indicates the thermal stability of the magnetoresistive properties of thin-film alloys based on Py and Cu.
ACKNOWLEDGMENT
This work was partially funded by the State Program of the Ministry of Education and Science of Ukraine No 0120U102005 (2020-2022 years).
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Структурно-фазовий стан та магнітотранспортні властивості тонкоплівкових сплавів на основі пермалою та міді
І.О. Шпетний1, К.В. Тищенко1, В.Я. Пак1, В.I. Дужий2, Ю.О. Шкурдода1, І.Ю. Проценко1
1 Сумський державний університет, вул. Римського-Корсакова, 2, 40007 Суми, Україна 2 Харківський національний аерокосмічний університет «Харківський авіаційний інститут»,
вул. Чкалова, 17, 61070 Харків, Україна
У роботі представлені результати досліджень впливу складу на структурно-фазовий стан та магніторезистивні властивості свіжосконденсованих та термооброблених при температурах Tв ≤ 900 К зразків плівкових сплавів на основі Py та Cu. Зразки тонкоплівкових сплавів товщиною d 25 нм в інтервалі складів 19 ≤ cCu ≤ 69 (де cCu – загальна концентрація Cu, ат.%) були отримані методом одночасного випаровування у вакуумі з двох незалежних випарників. Методом просвічуючої електронної мікроскопії було досліджено структурно-фазовий стан зразків плівкових сплавів при cCu 19 ат.%, 34 aт.% та 61 ат.%. Структура тонких плівок як в свіжесконденсованому так і у відпаленому при Tв ≤ 900 К стані складаються з квазігранул пермалою, вбудованих у немагнітну матрицю Cu. Фазовий стан зразків при cCu 19 ат.% та cCu 34 ат.% у свіжосконденсованому стані та після термообробки при Tв 600 К відповідав ГЦК-NixFe(x ≈ 3) + ГЦК-Cu. Після термообробки при температурах 700 Tв 900 К фазовий стан зразків при cCu 19 ат.% та cCu 34 ат.% відповідав Ni3Fe + ГЦК-Cu. Для зразка плівкового сплаву при cCu 61 ат.% у свіжесконденсованому стані та після термообробки при температурах 600 Tв 900 К фазовий стан відповідав ГЦК-NixFe (x ≈ 3) + ГЦК-Cu. Дослідження магніторезистивних властивостей плівкових зразків показали, що плівкові зразки у всьому інтервалі складів 19 ≤ cCu ≤ 69 aт.% характеризувалися ізотропним магнітоопором. Максимальне значення гігантського магнітоопору спостерігалося для зразка з cCu 34 aт.% як у свіжосконденсованому стані так і після термообробки при температурах 600 Tв 900 К. Термообробка зразків в інтервалі температур 600 Tв 900 К майже не вплинула на величину ГМО плівок при 19 ≤ cCu ≤ 51 aт.%.
Ключові слова: Ефект ГМО, Спін-залежне розсіювання, Пермалой, Суперпарамагнетизм.
01020-6
ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Том 13 № 1, 01020(6cc) (2021)
Structural-Phase State and Magnetotransport Properties of Thin Film Alloys Based on Permalloy and Copper
I.O. Shpetnyi1,*, K.V. Tyschenko1, V.Ya. Pak1, V.I. Duzhyi2, Yu.O. Shkurdoda1, I.Yu. Protsenko1
1 Sumy State University, 2, Rimsky-Korsakov St, 40007 Sumy, Ukraine 2 National Aerospace University "Kharkiv Aviation Institute", 17, Chkalov St, 61070 Kharkiv, Ukraine
(Received 03 January 2021; revised manuscript received 15 February 2021; published online 25 February 2021)
The paper presents the results of research influence of composition on structural-phase state and magnetoresistive properties of as-deposited and heat-treated at temperatures Ta ≤ 900 K samples film alloys based on Py and Cu. Samples of thin film alloys with a thickness of d 25 nm in the range of 19 ≤ cCu ≤ 69 (where cCu is the concentration of Cu, at.%) were obtained by the method of co-evaporation in vacuum from two independent evaporators. The structural-phase state of the samples of film alloys at cCu 19 at.%, 34 at.% and 61 at.% was investigated by the method of transmission electron microscopy. The structure of thin films in both as-deposited and annealed at Ta ≤ 900 K state consists of quasi granules with permalloy embedded in a nonmagnetic Cu matrix. The phase state of the samples at cCu 19 at.% and cCu 34 at.% in the as-deposited state and after heat treatment at Ta 600 K corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu. After heat treatment at temperatures of 700 Ta 900 K, the phase state of the samples at cCu 19 at.% and cCu 34 at.% corresponded to Ni3Fe + fcc-Cu. For the film alloy sample at cCu 61 at.% in the as-deposited state and after heat treatment at temperatures of 600 Ta 900 K, the phase state corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu. Studies of the magnetoresistive properties of film samples showed that the film samples in the entire range of compositions of 19 ≤ cCu ≤ 69 at.% were characterized by isotropic magnetoresistance. The maximum value of the giant magnetoresistance was observed for the sample with cCu 34 at.% Both in the as-deposited state and after heat treatment at temperatures of 600 Ta 900 K. Heat treatment of samples in the temperature range of 600 Ta 900 K had almost no effect on the value of GMR films at 19 ≤ cCu ≤ 51 at.%.
Keywords: GMR effect, Granular state, Spin-dependent scattering, Permalloy, Superparamagnetism.
DOI: 10.21272/jnep.13(1).01020
PACS numbers: 61.05.J –, 61.72. – y, 75.47.De, 81.40.Gh
1. INTRODUCTION
Thin metal film materials are characterized by a number of phenomena, the study of which remains relevant today [1-6]. One such effect is the phenomenon of giant magnetoresistance (GMR) in multilayers and granular film alloys [7-10]. The discovery of the GMR phenomenon has aroused widespread interest in these nanostructures due to the possibility of their application in spintronics, sensor electronics, biotechnology, medicine, instrumentation and other fields [1, 2, 11-15]. The effect of giant magnetoresistance is observed in granular magnetic films based on alloys of Co-Ag, Fe-Ag, Co-Cu, Permalloy-Ag, Permalloy-Cu, where magnetic granules with a size of units up to 100 nm are randomly placed in the volume of the nonmagnetic matrix [13, 14, 16-18]. The GMR effect is the result of spin-dependent scattering of conduction electrons at the interface between the nonmagnetic matrix and the magnetic granule or in the volume of the magnetic granule [6, 17]. When making such samples in a magnetic field, their resistivity shows large changes. It is established that the physical properties of granular magnetic film materials are determined by their composition and, accordingly, the structural-phase state. The magnetic, magnetoresistive, magneto-optical and electrical properties can be controlled by the change of the film alloy composition, and accordingly affecting the size and concentration of magnetic granules in the nonmagnetic matrix [16, 19]. Nowadays, the efforts of researchers are aimed at developing new film systems
with specified physical properties and ensuring the stability of these properties under the action of various factors, including heat treatment.
To date, a large number of experimental results of the study of magnetic and magnetoresistive properties of multilayers and multilayer film systems based on permalloy (Py) and Cu [17, 20-23]. Py and Cu-based film systems are widely used due to certain unique features low values of coercivity and saturation magnetization [1], thermal stability [21, 22], high sensitivity to the magnetic field [20]. Due to these properties, magnetic film systems have found practical application in biomedical technologies in the manufacture of biosensors [1], in automobile electronics [1], in spintronics in the manufacture of sensitive elements of magnetic field sensors [2], magnetic transducers, magnetoresistive random access memory [24]. In the works [21, 22] the authors studied the effect of annealing of multilayers based on permalloy and copper on their magnetic and magnetotransport properties, analyzed the mechanism of degradation of samples. However, studies of these effects for thin-film alloys in a wide range of compositions based on Py and Cu, obtained by the method of simultaneous condensation of two independent evaporators in the literature are almost absent. The aim of this work was to study the influence of the composition of film samples based on Py and Cu with a thickness d 25 nm on their structural-phase state and magnetoresistive properties under heat treatment.
* [email protected] 2077-6772/2021/13(1)01020(6)
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2021 Sumy State University
I.O. SHPETNYI, K.V. TYSCHENKO, V.YA. PAK ET AL.
2. EXPERIMENTAL DETAILS
Samples of thin-film alloys based on permalloy and
copper in a wide range of 19 ≤ сCu ≤ 69 (where сCu is the
total copper concentration, at.%) were obtained in a
vacuum of P ≈ 10 – 4 Pa at room temperature. To obtain
these film samples used the method of simultaneous
evaporation of metals from two independent electron
beam evaporators. The starting material for spraying
Py was a massive portion of Ni80Fe20 permalloy. The
deposition rate was 0.2 nm/s for Py and 0.3 nm/s for
Cu. The thickness of the film samples during the depo-
sition process was controlled by the quartz resonator
method [24]. Polished sieve substrates were chosen for
deposition. All the obtained samples were obtained in
one deposition cycle, therefore, they had the same
thickness d 25 nm, but differed in the ratio of the
concentration of the components. Annealing of samples
at temperatures Ta 600, 700 and 900 K was carried
out in a vacuum chamber at P 10 – 4 Pa for a time
t 30 min. The elemental composition of the obtained
thin film samples was examined using scanning elec-
tron microscopy (SEM Jeol 7001TTLS) and energy-
dispersion X-ray (EDX) spectroscopy (Oxford Instru-
ments analyzer, XMax detector). An accelerating volt-
age of 10 kV and an operating distance of 10 mm were
used to obtain EDX spectra.
The structural-phase state of the samples was in-
vestigated by transmission electron microscopy (TEM)
on the device TEM-125K (accelerating voltage 100 kV).
Preparation of samples for research by the TEM meth-
od was carried out by deposition of permalloy and cop-
per on a grid with a carbon film. Samples for research
by TEM were obtained simultaneously with the sam-
ples for the study of magnetic transport properties, ie
were identical.
The study of magnetoresistive properties was car-
ried out at room temperature on an automatic measur-
ing system in two geometric dimensions – transverse
and longitudinal [26]. Measurements were performed
by applying a magnetic field with an amplitude in the
range Bmax ± 0.45 T using a 4-point measurement
scheme. In the transverse geometry of the measure-
ment, the applied magnetic field was directed in the
plane of the film, but perpendicular to the direction of
the current I. In the longitudinal geometry of the
measurement, the magnetic field was directed in the
plane of the film and parallel to the direction of the
current I [25]. GMR values were calculated by the ratio
GMR ∆R/R(Bmax) (R(B) – R(Bmax))/R(Bmax)
[20],
where R(B) and R(Bmax) are the resistance of the films
in an arbitrary magnetic field B and in the maximum
field Bmax, respectively.
3. RESULTS AN DISCUSSION
3.1 Structural State and Phase Composition
The results of studies of the structural-phase state of film samples are very important for the interpretation of their magnetoresistive properties. The structural-phase state of samples of film alloys with copper concentrations of 19 at.%, 34 at.% and 61 at.% was analyzed by TEM method in as-deposited state and after
J. NANO- ELECTRON. PHYS. 13, 01020 (2021)
heat treatment. It should be noted that according to [27], in the massive and film states in permalloy alloys, depending on the ratio between the concentrations of Ni and Fe, three crystalline phases can be formed. At cNi 6385 at.%, the phase composition of Py films corresponds to fcc-Ni3Fe (structural type Cu3Au) with the lattice parameter a 0.3540.359 nm. At concentrations of cNi 50 at.% in the films, the fcc-Ni-Fe phase (structural type CuAu) with the lattice parameter a 0.3590.361 nm is stabilized. At cNi 25, the bcc -Fe-Ni phase with the lattice parameter a 0.286 nm is stabilized.
In Figs. 1 and 2 microstructure images and diffraction spectra are presented for a film sample with a to-
tal copper concentration cCu 19 at.% in as-deposited
state and after heat treatment at 600 and 700 K. On the inserts Fig. 1a, b the corresponding diffraction patterns for film samples are given. The diffraction pattern was transformed into a spectrum, by software created in the programming language Lab View. On Fig. 2 diffraction spectra for the images of diffraction pattern presented on Fig. 1. Vertical lines on Fig. 2 indicate the tabular data of the diffraction lines for massive samples of fcc-Cu, fcc-Ni-Fe and fcc-Ni3Fe. The decoding of diffraction patterns was performed taking into account the positions of the peaks in the diffraction spectra for as-deposited and heat-treated samples.
Fig. 1 – Microstructure of thin-film alloy based Py and Cu with a concentration of copper cCu 19 at.% in as-deposited state (a) and after heat treatment at 700 K (b). The inserts show the corresponding diffraction patterns
Fig. 2 – Diffraction spectra from samples of alloy thin films based on Py and Cu with a total concentration of cCu 19 at.% in as-deposited state and after heat treatment at different temperatures Ta
Analysis of the research results showed that the phase state of this sample in the as-deposited state corresponds to fcc-NixFe (x ≈ 3) + fcc-Cu. The size of the quasi granules (G) permalloy was L 7÷18 nm (Fig. 1a). Lines of fcc-Cu and fcc-NixFe, cannot be separated on the diffraction pattern due to close interplanar distances. Deviations from the composition Ni3Fe to NixFe (x ≈ 3) may caused
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STRUCTURAL-PHASE STATE AND MAGNETOTRANSPORT PROPERTIES…
J. NANO- ELECTRON. PHYS. 13, 01020 (2021)
by the dissociation of Ni3Fe permalloy to Ni and Fe atoms during the process of electron beam evaporation. A phase can be formed from these atoms fcc-Ni-Fe. In addition, the diffusion of Ni and Fe atoms may result in the formation of dilute solid solutions fcc-Cu (Ni) and fcc-Cu (Fe). After heat treatment of the film sample with the total concentration of copper cCu 19 аt.% at Ta 700 K the phase state corresponded fcc-Cu + fcc-Ni3Fe. During heat treatment of samples due to thermal diffusion of Ni atoms and their attachment to the granules NixFe Ni3Fe stoichiometry is renewed as in the material of the original sample. The size of the quasi granules was L 7÷23 nm (Fig. 1b).
Structural-phase state of the film sample with the total concentration of copper cCu 34 аt.% in the asdeposited state and after heat treatment (Fig. 3 and 4) similar to the state of the sample at cCu 19 at.%. Asdeposited films were characterized by a state in which quasi granules fcc-(NixFe) (x ≈ 3) size L 7÷15 nm (Fig. 3a) were in a nonmagnetic matrix with fcc-Cu. The results of decoding diffraction patterns are presented in Table 1. Annealing at Ta 600 K did not cause changes in phase state (Fig. 3b). The size of the quasi granules was L 8÷16 nm. After heat treatment of the film samples at Ta ≥ 700 K, the phase state corresponded fcc-Ni3Fe + fcc-Cu. Annealing of the samples at Ta 700 K and Ta 900 K increased the size of the quasi granules with permalloy to L 8÷21 nm (Fig. 3c) and L 27 ÷ 43 nm, respectively.
magnetic matrix. The size of the granules in asdeposited and heat-treated at 600, 700 and 900 K states were L 6÷10 nm (Fig. 1a), L 8÷12 nm (Fig. 1b), L 8÷19 nm (Fig. 1c) and L 8÷23 nm (Fig. 1d), respectively.
Fig. 4 – Diffraction spectra from samples of alloy thin films based on Py and Cu with a total concentration of cCu 34 at.% in as-deposited state and after heat treatment at different temperatures Ta
Fig. 3 – Microstructure of thin-film alloy based Py and Cu with a concentration of copper cCu 34 at.% in as-deposited state (a) and after heat treatment at 600 (b), 700 (c) and 900 K (d). The inserts show the corresponding diffraction patterns
Analysis of the phase composition of the film sample at cCu 61 at.% in as-deposited state and after heat treatment at temperatures of 600 Ta 900 K corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu (Fig. 5). The results of decoding the diffraction spectra (Fig. 6) are given in Table 1. No renewal NixFe to the stoichiometry of Ni3Fe occurred during heat treatment of the samples caused by to the thermal diffusion of Ni atoms, possibly because of the low total concentration of the magnetic component and, accordingly, the Ni atoms in the sample.
The structures of thin films in both as-deposited and annealed at Ta 900 K states consist of permalloy fcc-NixFe quasi granules embedded in a Cu non-
Fig. 5 – Microstructure of thin-film alloy based Py and Cu with a concentration of copper cCu 61 at.% in as-deposited state (a) and after heat treatment at 600 (b), 700 (c) and 900 K (d). The inserts show the corresponding diffraction patterns
3.2 Magnetoresistive Properties
The results of the study on the magnetoresistive properties in as-deposited state and after heat treatment alloys thin films based on Py and Cu are shown below. On Fig. 7 the dependence of the GMR value on the total concentration of copper in the film alloy is given. Magnetoresistance studies were performed in the transverse (Fig. 7a) and longitudinal (Fig. 7b) measurement geometries at room temperature in an external magnetic field B 0.45 T. At low total concentrations of copper in the alloy (for example, when cCu 19 at.% and cCu 26 at.%) quasi granules with permalloy, can touch each other, form a normal ohmic conduction channel. Therefore, in this case, the spindependent scattering of conduction electrons is inefficient, and the amplitude of the GMR is correspondingly small (GMRmax ≈ 0.1 % in the as-deposited state for the sample with cCu 19 at.%).
The increasing total copper concentration to cCu 34 at.% rises the amplitude of GMRs up to 0.21 %,
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I.O. SHPETNYI, K.V. TYSCHENKO, V.YA. PAK ET AL.
J. NANO- ELECTRON. PHYS. 13, 01020 (2021)
Fig. 6– Diffraction spectra from samples of alloy thin films based on Py and Cu with a total concentration of cCu 61 at.% in as-deposited state and after heat treatment at different temperatures Ta
the maximum value for as-deposited samples. In Fig. 8 field dependences for a sample of a film sample on the basis of permalloy and copper at the general concentration of copper are resulted cCu 34 at.%. The lack of saturation on the field dependences of the magnetoresistance (Fig. 8) indicates that the structural state of this sample corresponds to the superparamagnetic granules placed in a nonmagnetic matrix. Heat treatment of a sample with a total concentration of copper
cCu 34 at.% at temperature Ta 600 K led to an increase in the value of GMRs by 38 % and 14 % of the value obtained for as-deposited samples in the transverse and longitudinal geometry of the measurement, respectively. However, further heat treatment of the sample in the temperature range of 600 Ta 900 К almost did not change the value of its magnetoresistance (Figs. 7 and 8).
Further increase total copper concentration to 51 69 at.% caused reducing the amplitude of the magnetoresistance to 0.02-0.05 % in an external magnetic field of 0.45 T. The field dependences of the magnetoresistance of such samples are linear, do not show hysteresis and are not saturated in fields up to 0.45 T. This behavior of the magnetoresistance indicates that the structural state of these films corresponds to the ensemble of weakly interacting superparamagnetic granules. This is confirmed by the results of the study of the structural state, performed by the TEM method.
It should be noted that for film samples with a total concentration of copper 19 cCu 51 at.% the thermal stability of the magnetoresistance was observed. This feature of the magnetic transport properties of film systems based on Py and Cu can be used in the manufacture of instrument structures. Thermal stability of magnetoresistive properties was also observed by the authors in multilayers based on permalloy and copper [22].
Table 1 – Deciphering the diffraction patterns from Py and Cu alloy films with concentration of Cu 34 at.% and 61 at.% in asdeposited state and after heat treatment
No I,
dhkl,
hkl Phase state No I,
dhkl,
hkl Phase state
%
nm
%
nm
cCu 34 at.%
As-deposited sample
After annealing at Ta 900 K
1 100 0.2066 111 fcc-Cu G. fcc-NixFe
1 100 0.2040 111 fcc-Cu G. fcc-Ni3Fe
2 40 0.1808 200 fcc-Cu
2 65 0.1774 200 fcc-Cu
G. fcc-NixFe
G. fcc-Ni3Fe
3 20 0.1268 220 fcc-Cu
3 40 0.1251 220 fcc-Cu
G. fcc-NixFe
G. fcc-Ni3Fe
4 20 0.1084 311 fcc-Cu
4 40 0.1070 311 fcc-Cu
G. fcc-NixFe
G. fcc-Ni3Fe
a 0.359 ± 0.001 nm
a 0.355 ± 0.001 nm
cCu 61 at.%
As-deposited sample
After annealing at Ta 700K
1 100 0.2076 111 fcc-Cu G. fcc-NixFe
2 30 0.1808 200 fcc-Cu G. fcc-NixFe
3 20 0.1269 220 fcc-Cu G. fcc-NixFe
4 20 0.1089 311 fcc-Cu G. fcc-NixFe
a 0.360 ± 0.001 nm
1 100 0.2072 111 fcc-Cu G. fcc-NixFe
2 45 0.1798 200 fcc-Cu G. fcc-NixFe
3 30 0.1269 220 fcc-Cu G. fcc-NixFe
4 35 0.1083 311 fcc-Cu G. fcc-NixFe
a 0.359 ± 0.001 nm
Table values dhkl, nm
fcc-Cu 0.2080
fccNi-Fe
0.2060
fccNi3Fe
0.2044
0.1798 0.1783 0.1772
0.1271 0.1259 0.1253
0.1083 0.1073 0.1069
fcc-Cu 0.2080 0.1798 0.1271 0.1083
fccNi-Fe 0.2060
0.1783
0.1259
0.1073
fccNi3Fe 0.2044
0.1772
0.1253
0.1069
It should be noted that studies of the magnetoresistive properties of the samples showed that film alloys based on Py and Cu in the entire studied range of compositions 19 ≤ сCu ≤ 69 at.% In the as-deposited state and after heat treatment to Ta 900 K were characterized by an isotropic magnetoresistance. It means, the behavior of the magnetoresistance curves obtained in the transverse and longitudinal geometries of the measurement had a similar character.
4. CONCLUSIONS
1. The correlation between the composition, structural-phase state and magnetoresistive properties of samples of film alloys based on Py and Cu with a thickness of d 25 nm in the as-deposited state and after heat treatment at Ta 900 K was experimentally confirmed.
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STRUCTURAL-PHASE STATE AND MAGNETOTRANSPORT PROPERTIES…
J. NANO- ELECTRON. PHYS. 13, 01020 (2021)
Fig. 7 – The magnetoresistance in transverse (a) and longitudinal (b) geometries of nanogranular alloy thin films based on Py and Cu as a function of Cu atomic concentration, in as-deposited state and after annealing at different temperatures Ta. The measurements were performed at room temperature in magnetic field Bmax 0.45 T
Fig. 8 – The magnetoresistive curves measured in transverse (a) and longitudinal (b) geometries at the room temperature for the as-deposited and annealed at different temperatures nanogranular alloy thin films based on Py and Cu with a total concentration of cCu 34 at.%
2. The structural-phase state of samples of film alloys at сCu 19 at.%, 34 at.% and 61 at.% was investigated by TEM method. The structure of film systems in the as-deposited state and after heat treatment at
T 900 K consists of quasi-permalloy granules embedded in a nonmagnetic Cu matrix. Phase state of samples at cCu 19 at.% and cCu 34 at.% in as-deposited state and after heat treatment at Ta 600 K corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu. After heat treatment at temperatures 700 Ta 900 K phase state of the samples with concentrations of cCu 19 at.% and cCu 34 at.% corresponded to Ni3Fe + fcc-Cu. For a sample of a film alloy at cCu 61 at.% In the asdeposited state and after heat treatment at temperatures of 600 Ta 900 K phase state corresponded to fcc-NixFe (x ≈ 3) + fcc-Cu.
3. Studies of the magnetoresistive properties of the measured samples showed that film systems based on Py and Cu in the entire range of compositions 19 ≤ сCu ≤ 69 at.% are characterized by isotropic magnetoresistance. The maximum value of the giant magnetoresistance was observed for a sample with a total copper concentration сCu 34 at.% as in as-deposited condition and after heat treatment at temperatures 600 Ta 900 K.
4. Heat treatment of film samples with total concentration copper 19 ≤ сCu ≤ 51 at.% in the temperature range 600 Ta 900K had almost no effect on the value of GMRs. It indicates the thermal stability of the magnetoresistive properties of thin-film alloys based on Py and Cu.
ACKNOWLEDGMENT
This work was partially funded by the State Program of the Ministry of Education and Science of Ukraine No 0120U102005 (2020-2022 years).
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Структурно-фазовий стан та магнітотранспортні властивості тонкоплівкових сплавів на основі пермалою та міді
І.О. Шпетний1, К.В. Тищенко1, В.Я. Пак1, В.I. Дужий2, Ю.О. Шкурдода1, І.Ю. Проценко1
1 Сумський державний університет, вул. Римського-Корсакова, 2, 40007 Суми, Україна 2 Харківський національний аерокосмічний університет «Харківський авіаційний інститут»,
вул. Чкалова, 17, 61070 Харків, Україна
У роботі представлені результати досліджень впливу складу на структурно-фазовий стан та магніторезистивні властивості свіжосконденсованих та термооброблених при температурах Tв ≤ 900 К зразків плівкових сплавів на основі Py та Cu. Зразки тонкоплівкових сплавів товщиною d 25 нм в інтервалі складів 19 ≤ cCu ≤ 69 (де cCu – загальна концентрація Cu, ат.%) були отримані методом одночасного випаровування у вакуумі з двох незалежних випарників. Методом просвічуючої електронної мікроскопії було досліджено структурно-фазовий стан зразків плівкових сплавів при cCu 19 ат.%, 34 aт.% та 61 ат.%. Структура тонких плівок як в свіжесконденсованому так і у відпаленому при Tв ≤ 900 К стані складаються з квазігранул пермалою, вбудованих у немагнітну матрицю Cu. Фазовий стан зразків при cCu 19 ат.% та cCu 34 ат.% у свіжосконденсованому стані та після термообробки при Tв 600 К відповідав ГЦК-NixFe(x ≈ 3) + ГЦК-Cu. Після термообробки при температурах 700 Tв 900 К фазовий стан зразків при cCu 19 ат.% та cCu 34 ат.% відповідав Ni3Fe + ГЦК-Cu. Для зразка плівкового сплаву при cCu 61 ат.% у свіжесконденсованому стані та після термообробки при температурах 600 Tв 900 К фазовий стан відповідав ГЦК-NixFe (x ≈ 3) + ГЦК-Cu. Дослідження магніторезистивних властивостей плівкових зразків показали, що плівкові зразки у всьому інтервалі складів 19 ≤ cCu ≤ 69 aт.% характеризувалися ізотропним магнітоопором. Максимальне значення гігантського магнітоопору спостерігалося для зразка з cCu 34 aт.% як у свіжосконденсованому стані так і після термообробки при температурах 600 Tв 900 К. Термообробка зразків в інтервалі температур 600 Tв 900 К майже не вплинула на величину ГМО плівок при 19 ≤ cCu ≤ 51 aт.%.
Ключові слова: Ефект ГМО, Спін-залежне розсіювання, Пермалой, Суперпарамагнетизм.
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